The present invention teaches a vacuum-compatible die bonding technique suitable for mounting die within a vacuum housing for use in a ultra high vacuum (UHV) tube. An application where this invention is particularly valuable is when used with charge coupled devices (CCDs) or CMOS in a vacuum environment in which the semiconductor die (CCD or CMOS) is directly electron bombarded in an imaging system as, for example, as is disclosed in U.S. Pat. No. 4,687,922 (CCDs) or as disclosed in U.S. Pat. No. 6,285,018 B1 (CMOS). This invention is also applicable for attaching hybrid read out integrated circuit devices (ROIC) to thermally mismatched substrates such as ceramics within photocathode based vacuum tubes.
In the prior art, as described for example in U.S. Pat. No. 6,281,572, an intermediate pedestal is used between mismatched materials. The pedestal makes use of both an intermediate thermal coefficient of expansion (TCE) and a restricted area in order to limit the TCE mismatch induced strain in the finished device. The induced strain in the finished device is roughly proportional to the TCE mismatch times the change in temperature from the point at which the braze material solidified times the linear dimension of the braze. In the prior art, a braze material was chosen that remained in it's solid form throughout the tube vacuum process cycle. Typical braze materials and melting temperatures included AuSn (˜280° C.) and AuGe (˜361° C.) alloys. Modern photocathode based sensors including semiconductor devices are typically sealed under ultra-high vacuum (UHV) conditions. In order to achieve these conditions, tube components are typically baked at temperatures in excess of 200° C. in vacuum to eliminate residual gasses from tube components. Typically, the brazes are aligned using fixtures in order to accurately set the position of the semiconductor device within the tube. The fixtures are later removed and the tube is run through a UHV seal cycle. The high temperature of the braze alloys is required in the prior art to insure that the semiconductor does not move during the vacuum sealing step. The downside of the high temperature braze is that it locks in a high degree of strain in the finished assembly when it is cooled to room temperature. Another approach used in the prior art is to simply minimize the maximum linear dimension of the braze pad in order to minimize strain. A 0.050″ braze pad has been shown to work reasonably well with an AuSn braze. U.S. Pat. No. 6,507,147 describes a prior art package suitable for UHV applications that makes use of a small area braze. There is however a downside to this approach too. The use of a small pad limits the heat transfer area between the semiconductor device and the underlying substrate. In the case of electron bombarded CMOS image sensors or ROIC based CMOS anodes, heating of the sensor resulting from dissipation in the chip can and generally will result in degraded sensor performance. These difficulties in the prior art are avoided and/or considerably reduced through the use of physically compliant vacuum-compatible die bonding techniques enabling the efficient mounting of die within the vacuum area in a UHV tube.
In prior art UHV tubes such as the one described in U.S. Pat. No. 6,281,572, electrical connection between the semiconductor device and the inside of the vacuum envelope has been made through the use of wire-bonds that extend from the exposed surface of the semiconductor die to the underlying surface of the vacuum package. The use of wire-bonds is a well-established, reliable way to make multiple electrical connections between a semiconductor device and a thermal coefficient of expansion mismatched package. The down side of this approach is the additional package volume dedicated to the annular ring of pads that lie outside the projected outline of the overlying semiconductor die. Through the use of low melting point physically compliant braze materials, the connection pads can be moved below the semiconductor die, thereby conserving package volume and projected area.